cdrb tx: remove libpq specific error

[drakkan]

libpq implements SQLState() string method as pgx since 1.10.6.

I think we could also replace the remaining lib/pq imports with pgx in the future. Fow now is better to keep them and require lib/pq 1.10.6

[cockroach-teamcity]

This change is 

[drakkan]

This patch must be merged after #132 I just applied the changes there to verify that tests pass. I'll now revert that changes using a force-push

[rafiss]

hm, will this break anything for users who do not upgrade to the new version of lib/pq?

i wonder if we'd need a new major release if so

[drakkan]

hm, will this break anything for users who do not upgrade to the new version of lib/pq?

i wonder if we'd need a new major release if so

lib/pq is still imported here and in the testserver, so it is still in the go.mod.

I bumped the version in go.mod to 1.10.6, so this should not be a breaking change.
In the future we should replace these imports with pgx and so lib/pq will be removed from go.mod and this will be a breaking change suitable for a major version

[rafiss]

i see. thanks for your contribution!